The present invention is directed in general toward local area networks (LANs) and, in particular, to token passing LANs. Local area networks are becoming more and more commonplace in today's society and with this widespread acceptance and usage of these networks comes the necessity to improve their overall speed and efficiency. Many factors are taken in account when judging the effectiveness of LANs. Among these factors are bus or waiting time which, as used herein, refers to the length of time a bus interface unit (BIU) must store a particular message in its buffer memory before gaining access to the bus for transmission of that message.
Another factor is bus utilization which refers to the percentage of time the transmission bus is being used for the transmission of data between users. Bus utilization does not include the percentage of time the bus is being used for transmission of signals to implement the network protocol, e.g., transmission of the token. Bus waiting time increases and bus utilization is decreased, when the token is passed to BIUs having no messages, or data, to transmit.
Also to be considered as an environment of LANs is the packet arrival rate which refers to the rate at which messages arrive at the bus interface unit to be transmitted in the network.
Still another factor to be evaluated in judging the effectivenss of LANs is BIU service time which is used herein to refer to the time which passes between successive opportunities for a given BIU to place a message on the transmision bus.
Yet another factor used in judging the operation of a LAN is token rotation time which is used herein to refer to the time it takes the token to traverse the LAN.
Token passing local area networks may be either static or dynamic. Dynamic token passing local area networks are those in which stations may enter and exit the logical ring--the list which prescribes the order in which the token is passed--and thereby may elect to participate in the passing of the token or not. Static token passing local area networks are those in which each bus interface unit of the local area network is always participating in the passing of the token.
Dynamic token passing local area networks improve their bus waiting times and bus utilization by allowing a BIU having no messages to transmit to exit the logical ring. Thus, only active stations participate in passing the token and, consequently, bus waiting time is decreased while bus utilization is increased. Many of these networks, however, incur delays, and therefore increase bus waiting time, in re-establishing the logical ring each time a BIU either exits or enters the ring. Additionally, these networks are subject to extreme increases in mean bus waiting times as the packet arrival rate increases.
Conversely, while static local area networks do not incur delays for entering and exiting stations, these systems require that each station receive the token whether or not that station has messages to transmit. Thus, these systems incur unacceptable delay in bus waiting time during periods where relatively few stations have messages to transmit. Further, like the dynamic token passing network, these systems are subject to extreme increases in mean bus waiting time as the packet arrival rate increases.
Lastly, while it may be advantageous in many appli-cations to employ a dynamic network, some applications are best served by a static network. It is desirable, therefore, to improve the overall bus waiting time of each of these networks.